Integrated starter generator (ISG) systems are widely used to apply torque to the rotating shaft of an engine at the startup of the engine.
An ISG system includes a motor-generator coupled to the rotating shaft of an engine via a belt, and causes the motor-generator as a starter to apply torque to the rotating shaft of the engine via the belt, thus starting, i.e. cranking, the engine. The ISG system also includes a starter motor, in addition to the motor-generator, for applying torque to the rotating shaft of the engine while the pinon of the starter motor is engaged with the ring gear of the rotating shaft of the engine at low temperatures. This is because the belt may be difficult to slide at the low temperatures, which may result in difficulty of smoothly applying torque to the rotating shaft of the engine via the belt.
The larger torque applied to the belt is, the higher strength and endurance of the belt need be. The larger toque applied to the belt may result in a belt tensioner provided for absorbing torque fluctuations.
In particular, Japanese Patent Publication No. 4421567, referred to as a published patent document, discloses such an ISG system, which includes both a motor-generator and a starter motor. The ISG system disclosed in the published patent document includes an electronic control system (ECU) that is programmed to cause the starter motor to apply a first torque to the rotating shaft of an engine until the occurrence of first firing, i.e. an initial ignition, in the engine. Thereafter, the ECU of the ISG system is programmed to cause the motor-generator to apply a second torque, which is lower than the first torque, to the rotating shaft of the engine until the engine is fired up in which the rotating shaft can be rotated by combustion operations of the engine itself. This enables the motor-generator to have relatively lower maximum output required to start the engine, thus reducing manufacturing cost of the ISG system.